Document how to setup a development environment

[HellRok]

I feel I should put in the effort to at least to a basic test on all major OS'

• [ ] Linux (Debian)
• [ ] Linux (Ubuntu)
• [ ] Linux (Fedora)
• [ ] Linux (Arch)
• [ ] OSX
• [ ] Windows

Things to remember:

• [ ] Use dx
• [ ] git hooks
• [ ] Maybe some doco around the file structure